Vacuum technology is more and more widely used in vacuum evaporation, crystallization, distillation, sublimation, drying, negative pressure concentration, dehydration, chemical reaction absorption and vacuum conveying of materials in petroleum, chemical industry, pharmaceuticals and other industries. But vacuum technology is a frontier science. China’s vacuum industry developed on the basis of digesting and absorbing products from the former Soviet Union, the United States, Japan and Germany after liberation. Although the number of vacuum equipment manufacturers in China has mushroomed in recent years, many manufacturers are not clear about how to apply and select vacuum pumps produced by them in different processes, which is very wide. The selection of large users brings great inconvenience.
Some Concepts about Vacuum Pumps
How to apply vacuum pump well in the process of chemical production needs to clarify the following concepts:
1. The concept of “vacuum”: “vacuum” refers to the state of a gas below an atmospheric pressure in a given space, that is, the gas molecular density in the space is lower than the atmospheric pressure in the region. The space state in which there is no gas at all is called absolute vacuum.
2. Vacuum pumps: devices used to generate, improve and maintain vacuum.
3. Pumping capacity: the flow rate of the suction gas per unit time under the working pressure of the vacuum pump. Unit: kg/h or m3/h;
4. Limit Pressure (Vacuum): The absolute pressure that a vacuum pump can achieve when the extraction capacity is zero. Units: Pa, KPa, MPa or mmHg (the local atmospheric pressure is the ultimate vacuum under extreme absolute pressure);
5. Working Pressure (Vacuum): Absolute Pressure of Jet Pump at Rated Pumping Volume. SETTING: Pa, KPa, MPa or mmHg (working vacuum for working absolute pressure reduction of local atmospheric pressure);
6. Working Steam Pressure: Working Steam Pressure of Steam Jet Vacuum Pump. Unit: MPa or kgf/cm2;
7. Working Steam Consumption: Steam Consumption per Unit Time of Steam Jet Vacuum Pump. Unit: kg/h;
8. Cooling water temperature: circulating water temperature of water jet pump or condenser circulating water temperature of steam jet vacuum pump. SETTING: OC;
9. Cooling water circulation: the circulating water flow through the condenser of water ejector or steam ejector pump per unit time. Unit: kg/h;
Several Vacuum Pumps in Chemical Industry and Their Advantages and Disadvantages
1. Several kinds of vacuum pumps commonly used in chemical industry-the common vacuum pumps in chemical industry can be simply divided into variable-volume vacuum pumps and jet vacuum pumps. Variable-volume vacuum pumps are devices that use the periodic variation of pump chamber volume to complete suction and exhaust. Reciprocating vacuum pumps, rotary vane vacuum pumps, slide valve vacuum pumps, water ring vacuum pumps and Roots vacuum pumps belong to variable-volume vacuum pumps. Type vacuum pump. Jet vacuum pump is a momentum transfer pump that uses the high-speed jet generated by the pressure drop of Venturi effect to transport gas to the outlet. Water jet vacuum pump, steam jet vacuum pump, steam-water series jet vacuum pump and steam-water combined jet vacuum pump belong to the same kind of jet vacuum pump.
2. Working Principle and Advantages and Disadvantages of Variable Volume Vacuum Pump
(1) Reciprocating vacuum pump, rotary vane vacuum pump, slide valve vacuum pump and Roots vacuum pump inhale, compress and discharge gas by reciprocating motion or rotation of piston. Their advantage is that the vacuum degree of the new pump is relatively high, but the piston is a running part, so the wear of the piston is inevitable. With the increase of the number of maintenance times, the working vacuum degree will continue to decline until it can not meet the production requirements. This kind of pump has the disadvantage of high noise, high fuel consumption and high failure rate. It generally needs to open a spare one, which increases the investment and operation cost of the equipment invisibly. In addition, if this kind of pump is used to pump condensable gases such as suction steam, lubricating oil will be emulsified, so it can only be used to suck non-condensable (air) gases, nor can it suck medium with granular shape, which limits its scope of application.
(2) Water ring vacuum pump rotates by an eccentric rotor with multi-blades installed in the pump shell, throwing water into the pump shell to form a water ring concentric with the pump shell, and the water ring and the rotor blades form a volume periodic change, thus inhaling, compressing and discharging gas. It has the advantages of large suction capacity at low vacuum and direct suction of condensable gases such as water vapor. Its shortcomings are low vacuum, inability to suck granular media, high-speed rotation of the rotor is not easy to do anti-corrosion treatment, so it can not suck corrosive media.
3. Jet Vacuum Pump Working Principle and Advantages and Disadvantages
Jet vacuum pump is a momentum transfer pump that uses the high-speed jet generated by the pressure drop of Venturi effect to transport gas to the outlet. It is divided into water jet vacuum pump, steam jet vacuum pump, steam-water series jet vacuum pump, steam-water combination jet vacuum pump. Jet vacuum pump has been widely used in chemical operation because of its wide vacuum range, direct suction of condensable gases such as suction steam and granular media, simple structure, easy operation, small maintenance of non-operating components, energy saving and consumption reduction. The working principles of various jet vacuum pumps are described below.
(1) Water Jet Vacuum Pump Working Principle and Structure
The equipment composition of the water jet vacuum pump shows its structure. Its working principle is that the water in the circulating water tank produces certain pressure and velocity after the circulating water pump works. Water with certain pressure and velocity enters the catchment chamber of the water ejector, and is ejected through several Laval nozzles on the orifice plate. The high-speed jet forms a vacuum in the mixing chamber of the ejector, and the medium is pumped under the vacuum action. Enter the ejector mixing chamber, fully mix with the high-speed water flow in the mixing chamber and discharge into the circulating water tank after the Venturi tube reduces speed and boosts pressure. The non-condensable gas is released, and the condensable vapor overflows from the overflow outlet of the water tank, so work is done repeatedly. It has the advantage of low-level whole machine type, higher vacuum than water ring vacuum pump. Replacing W-type reciprocating vacuum pump, it can cancel the pre-condenser and save one-time equipment investment and operation costs.
(2) Steam-water Series Jet Vacuum Pump Working Principle and Structure
In front of the ejector of the water jet vacuum pump, one or more steam jet vacuum pumps, i.e. steam-water series ejection vacuum pumps, are connected in series. The structure of the steam-water series ejection vacuum pumps can be seen. Its working principle is that saturated or superheated vapor at a certain pressure enters the mixing chamber of the steam ejector after decompression and acceleration through the Laval nozzle, which makes the mixing chamber produce vacuum, and the extracted medium is sucked into the mixing chamber to mix with the working vapor. After mixing, the fluid passes through the diffuser, the velocity drops and the pressure rises until it reaches the pressure of the next suction port and discharges into the next steam ejector pump. Or water jet pump. It is a type of low-level whole machine with relatively high vacuum, which can replace water ring-Roots unit and rotary blade-Roots unit to directly suck condensable gas, but it has limited amount of non-condensable gas in high vacuum.
(3) New High Efficiency Steam Jet Vacuum Pump Working Principle and Procedure
A new type of high efficiency steam jet vacuum pump is composed of one or more stage steam jet pumps and high efficiency condensers. Saturated or superheated vapor at a certain pressure enters the mixing chamber of the steam ejector after decompression and acceleration through the Laval nozzle. The mixing chamber produces vacuum, and the extracted medium is sucked into the mixing chamber to mix with the working vapor. The mixing fluid passes through the diffuser and the velocity decreases. The steam ejector is discharged into the next stage after reaching the pressure of the next stage suction port. After the mixed fluid discharged by the steam ejector in front of the condenser enters the high efficiency condenser, the condensable vapor body is cooled to liquid and non-condensable gas and discharged out of the condenser with the cooling water (see the process flow chart of the new high efficiency steam ejection vacuum pump). It can be installed at any height, and the amount of non-condensable gas in the medium suitable for suction is relatively small. Most of the process is condensable vapor body. It can save more than 60% working steam by replacing the traditional multi-stage steam jet vacuum pump.
(4) Steam-water Combined Jet Vacuum Pump Working Principle and Process Flow
It is composed of a steam-water jet vacuum pump in series behind the first condenser of a traditional steam jet pump. Its working principle can be seen in the flow chart of the steam-water combined jet vacuum pump. It overcomes the shortcomings of steam-jet vacuum pump and water-jet vacuum pump, and gives full play to their advantages. It can not only achieve a higher working vacuum, but also have a larger pumping capacity. Compared with the traditional multi-stage steam-jet vacuum pump, it can save more than 50% of working steam, 30% of cooling water, no need to equip a start-up pump, and the time of pumping tower is shorter.
(5) The advantages of the multi-stage steam jet vacuum pump are simple structure, convenient use, stable and reliable operation, but the disadvantages are large steam consumption (high operating costs), which must be installed above 11 meters height, and inconvenient maintenance.
Choosing reasonable vacuum pump in combination with technology
In the chemical production process, vacuum pumps can be reasonably selected according to the working pressure required by the specific process and the nature of the suction gas. Generally, the working pressure required by the specific process is determined first. The vacuum required by the process of vacuum distillation, distillation and sublimation is high (working pressure is low), and the vacuum is evaporated, dried, concentrated, crystallized, dehydrated, decolorized, absorbed by chemical reaction and true. Low vacuum (high working pressure) is required for air conveying materials and other processes. After the working pressure is determined, the vacuum pump can be reasonably selected according to the size of the pumping volume and the nature of the pumped gas.
1. List the scope of application of various vacuum pumps according to the working pressure of several vacuum pumps.
2. Pumping different gases and choosing different jet vacuum pumps
(1) Water jet vacuum pump is suitable for pumping various gases with working pressure greater than 25 mmHg (3.3 KPa), especially in the case of pumping condensable vapor (i.e. condensable gas in water or in case of condensation), dust and granular media. When the requirement of working pressure is relatively small and the amount of non-condensable gas is large, steam jet pump, steam-water series and steam-water combined jet vacuum pump should be selected.
(2) The working pressure of steam-water series jet vacuum pump can reach 2 mmHg (267 Pa), but it is smaller than the new high-efficiency steam jet vacuum pump, steam-water combined jet vacuum pump and multi-stage steam jet vacuum pump, which is suitable for the intermittent production of small and medium-sized enterprises.
(3) The working pressure of the new high-efficiency steam jet vacuum pump can reach 0.5mmhg (65Pa). Most of the gases suitable for suction are condensable gases, and the proportion of non-condensable gases is relatively small.
(4) The working pressure of steam-water combined jet vacuum pump and multi-stage steam-jet vacuum pump can reach less than 10 Pa, which can suck various gases. The difference between them is that steam-water combined jet vacuum pump saves working steam consumption and operation cost compared with multi-stage steam-jet vacuum pump.
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